A mass spectrometer (MS) generally includes an ion source for producing charged species from an introduced sample, a mass analyzer for separating the charged species according to their mass-to-charge ratios (m/z ratios, or simply “masses”), and an ion detector for counting the separated species to provide electrical signals from which mass spectra may be produced. The sample may be introduced into the ion source by various techniques. In one example, a gas chromatograph (GC) is interfaced with the MS such that the sample output from the GC column—containing chromatographically separated sample components (e.g., chemical compounds)—serves as the sample input into the ion source. The latter system is often termed a GC/MS system.
As an MS continues to be operated over time, invariably some alteration or degradation in the performance of the MS, particularly the ion source, occurs due to the samples, their matrix (e.g., heavy hydrocarbons in petroleum samples, triglycerides in fat samples, etc.) and solvents, stationary phase bleed from the GC column, or other recalcitrant substances, all of which may accumulate over time. Even at the initial operation of the MS, the MS may not be stabilized or “conditioned” to provide adequate or uniform performance. In the case of gas chromatography where an electron impact (EI) or chemical ionization (CI) source is typically utilized in the MS, the ion source can be rapidly fouled by the introduced sample components, which results in degraded performance as seen in the analyte signal or spectral characteristics. Another problem, especially with high-boiling analytes, is that peak tailing can increase with continued use in addition to reduced signal response. The degraded performance may be manifested in many ways, but typically the metrics are reduced analyte signal response and high system background noise, the latter being particularly troublesome for analyte detection and identification.
These problems have conventionally required that the MS be cleaned periodically. Generally, the higher the rate of contaminant deposition, the more often the MS must be cleaned. The common, conventional solution has been to vent the MS system, remove the critically affected components (e.g., ion source, ion optics, pre-filter, etc.), treat the removed components to mechanical and/or chemical cleaning followed by other processes (e.g., muffle or vacuum furnace baking), and then re-install the components in the MS system. Such conventional ex situ cleaning procedures can be quite complex and lengthy procedures, involving potentially toxic solvents, expensive equipment, and the time and care of skilled technicians. Moreover, the cleaning process only temporarily solves the problem. After performing an iteration of cleaning and resuming the analytical operation of the MS, the performance of the MS will start to degrade again, eventually requiring another iteration of cleaning. In addition, the conventional cleaning process may fail due to mechanical issues associated with the reinstallation of components, or because some step in the procedure was compromised (e.g., a cleaning solvent was contaminated). Such failures may not be discovered until the MS is reassembled, under vacuum, and at operating conditions. Also, the process of venting entrains certain background species, the most abundant of which is water, which results in additional time being required to eliminate these substances. Water as a contaminant can cause a rapid reduction in MS signal response.
U.S. Pat. Nos. 8,378,293 and 8,513,593, the entire contents of which are incorporated by reference herein, describe apparatuses and methods entailing the addition of a conditioning gas (or conditioning agent) to a mass spectrometry (MS) system to condition (or re-condition) the MS system in situ so as to improve or restore its performance. Hydrogen in particular was found to be highly effective as a conditioning agent in an MS environment. Hydrogen rapidly diffuses and displaces surface contaminants. Hydrogen when dissociated or in higher excited, meta-stable or pseudo-Rydberg states (such as by electron impact or other processes) is very active and can reduce many adsorbed compounds, such as those that tend to become adsorbed on ion source surfaces and degrade operation. Moreover, hydrogen can alter metal oxidation states. The metal surfaces of an MS system are known to participate in a variety of reactions that affect the analytes and other introduced compounds, such as dehydration or reduction as occur in the ion source. By converting the metals from a range of oxidation states to a reproducible and fixed set, performance can be made more consistent. In the case of the ion source, spectral characteristics can be greatly stabilized.
More recently, however, it has further been found that conditioning the MS system with hydrogen can be more favorable or less favorable for compound analysis by the MS system, depending on compounds themselves and the state of the MS system (e.g., the ion source) before and/or after treatment with hydrogen. That is, in some situations conditioning with hydrogen, or at least with hydrogen alone, may have adverse effects on the analysis of a given compound.
In view of the foregoing, there is an ongoing need in mass spectrometry, including chromatography/mass spectrometry, for further improvements in methods and apparatuses for conditioning an MS system. There is also a need for further improvements in methods and apparatuses for in situ conditioning that is carried out at the MS system, whereby the need for conventional ex situ cleaning is eliminated or at least significantly reduced and/or simplified.